The disclosure relates, in general, to the recovery of a target material from a geologic composition and, more particularly, to a system and method for the recovery of precious metals from a precursor type ore.
Gold (Au) is one of the most highly sought after elements mined from the earth and is used as currency, in commercial settings (e.g., jewelry, arts and crafts), and in industrial settings (e.g., electronics, medicine, optics). At standard conditions, gold exists as a solid, generally non-reactive chemical element. Gold can be found in nature as nuggets or grains, in rocks, in veins, in alluvial deposits, and in minerals as gold compounds.
Existing techniques for mining of gold ore can be economical with material containing as little as 0.5 ppm gold even though at this concentration, the gold may be invisible to the naked eye. Various concentrating techniques include leaching such as with an aqueous solution of cyanide to form a water soluble coordination complex according to the following reaction:4Au+8NaCN+O2+2H2O→4Na[Au(CN)2]+4NaOH.The gold can then be separated from the cyanide solution and the cyanide remediated to ammonia. More refractory gold containing ore may be subjected to various pretreatments including the application of heat, microbes, pressure, or mechanical grinding.
More recently, it has been discovered that various geologic materials may contain gold in a state that is undetectable using traditional analytical techniques such as inductively coupled plasma, atomic absorption spectroscopy, and fire assays/cupellation, the last of which is an accepted standard for valuing gold ore.
As described by van Deventer (Minerals Engineering 2013, vol. 53, pp. 266-275), a number of studies exist where fire assays were conducted on geologic materials such as mine tailings (i.e., gangue—the leftover ore material after separation from the valuable fraction of the ore), coal combustion products or CCPs (e.g., fly ash, flue-gas desulfurization materials, bottom ash, boiler slag) and other precursor-type ore materials. Generally, fire assaying such materials indicates that the samples contain an infinitesimal or undetectable amount of gold. In one study, Seredin et al. reported gold concentrations on the order of about 100 parts per billion (0.00001 wt %) in fly ash (Mineralium Deposita 2014, vol. 49, pp. 1-6). In another study, a fire assay was only able to identify about 0.01 ppm gold. However, after various treatment steps, it was possible to recover nearly 70 ppm gold from the sample. These results show that fire assay is an inaccurate and ineffective technique for determining the gold content of such geologic materials.
One possible explanation for the ineffectiveness of the aforementioned analysis techniques is that the gold may exist in a non-bulk physical state. It is well known that bulk materials tend to have a particular set of physical and chemical properties. In one aspect, the properties of bulk change as the scale decreases and the fraction of atoms at the surface of a material becomes significant. Thus, when atomic and nanoscale particles are isolated from the bulk material, the physical and chemical behavior of these particles can deviate. For example, gold nanoparticles are capable of forming colloidal suspensions as solvent-surface interactions overcome differences in density. Moreover, colloidal suspensions of submicron-sized gold particles can range in appearance from red to blue to purple depending on the particle size. Therefore, if the gold particles are present in the geologic material in a non-bulk physical state, it may not be possible to perform an accurate determination of the gold content of a sample using standard analytical techniques.
Given the commercial and industrial value of gold and the potential that there may be significant quantities of the element present in geologic materials including mine tailings and CCPs, it may be beneficial to recover the gold and other precious metals from these geologic materials. However, given that the precious metals may be present in a non-bulk physical state which is not amenable to traditional analytical techniques, there is a need for a system and a method to transform the precious metals into a bulk physical state so that they may be accurately detected and recovered in an economical fashion.